Methods and systems for using modulation frequencies for optical channel monitoring with periodic optical filtering

ABSTRACT

Methods and systems for using modulation frequencies for optical channel monitoring with periodic optical filtering may allocate modulation frequencies based on bandwidth slices of an optical spectrum. The bandwidth slices may have an equal spectral band that is associated with a respectively unique modulation frequency. An optical carrier aligning with a bandwidth slice may be modulated using the modulation frequency assigned to that bandwidth slice. Additionally, the bandwidth slices may be used to determine a maximum frequency error of an optical carrier when received, such as from frequency drift during transmission.

BACKGROUND

1. Field of the Disclosure

The present disclosure relates generally to optical communication networks and, more particularly, to methods and systems for using modulation frequencies for optical channel monitoring with periodic optical filtering.

2. Description of the Related Art

Telecommunications systems, cable television systems and data communication networks use optical networks to rapidly convey large amounts of information between remote points. In an optical network, information is conveyed in the form of optical signals through optical fibers. Optical networks may also include various network nodes such as amplifiers, dispersion compensators, multiplexer/demultiplexer filters, wavelength selective switches, couplers, etc. to perform various operations within the network.

Optical superchannels are an emerging solution for transmission of signals at 400 Gb/s and 1 Tb/s data rate per channel, and hold promise for even higher data rates in the future. A typical superchannel includes a set of subcarriers that are frequency multiplexed to form a single wavelength channel. The superchannel may then be transmitted through an optical network as a single channel across network endpoints. The subcarriers within the superchannel are tightly packed to achieve high spectral efficiency.

SUMMARY

In one aspect, a disclosed method is for using modulation frequencies for optical channel monitoring with periodic optical filtering. The method may include dividing an optical band into bandwidth slices, each of the bandwidth slices having an equal spectral band that is smaller than a minimum optical carrier bandwidth, and assigning, to each of the bandwidth slices, a modulation frequency that is respectively unique to each of the bandwidth slices. Based on a spectral position of a first bandwidth slice, the method may include assigning a first optical carrier aligned with the spectral position to a first modulation frequency corresponding to the first bandwidth slice. The method may also include transmitting the first optical carrier over an optical transport network, including modulating the first optical carrier using a first modulation frequency. In the method, the first optical carrier may be included with a plurality of optical carriers transmitted over the optical transport network. The method may further include receiving, from the optical transport network, the optical carriers transmitted, and scanning the optical carriers using a periodic optical filter over a free optical spectral range W_(fs) of the periodic optical filter to generate a filtered signal. In the method, the free optical spectral range W_(fs) may correspond to K number of optical carriers and K may be greater than or equal to 2. The method may further include demodulating the filtered signal to generate a power spectrum within the free spectral range for the first modulation frequency. Based on the first bandwidth slice and the power spectrum for the first modulation frequency, the method may still further include identifying a first center optical frequency f_(n) of the first optical carrier received.

In any of the disclosed embodiments of the method, when a first carrier frequency error for the first center optical frequency is greater than the equal spectral band, the method may include calculating a second center optical frequency f_(n′), and, based on f_(n′), calculating a second carrier frequency error. When the second carrier frequency error is less than the equal spectral band, the method may include determining that f_(n′) is an actual center optical frequency for the first optical carrier.

In any of the disclosed embodiments of the method, when a first carrier frequency error for the first center optical frequency is greater than the equal spectral band, the method may include calculating a third center optical frequency f_(n″), and, based on f_(n″), calculating a third carrier frequency error. When the third carrier frequency error is less than the equal spectral band, the method may include determining that f_(n″) is an actual center optical frequency for the first optical carrier.

In any of the disclosed embodiments of the method, a carrier frequency error for a carrier frequency f may be calculated using at least one of: an absolute value of a difference between f and a maximum frequency of the bandwidth slice corresponding to the modulation frequency f, and an absolute value of a difference between f and a minimum frequency of the bandwidth slice corresponding to the modulation frequency f.

In any of the disclosed embodiments of the method, the first optical carrier may be modulated using frequency modulation (FM), and the central alignment feature may be a relative minima in the power spectrum. In any of the disclosed embodiments of the method, the first optical carrier may be modulated using amplitude modulation (AM), and the central alignment feature may be a relative maxima in the power spectrum.

In any of the disclosed embodiments of the method, demodulating the filtered signal may include using a digital signal processor to detect at least the first modulation frequency. In any of the disclosed embodiments of the method, demodulating the filtered signal may include, when the first modulation frequency is not detected from the filtered signal, determining that the first optical carrier is not included in the optical carriers received. In any of the disclosed embodiments of the method, the optical carriers may be subcarriers of a superchannel, while the free optical spectral range W_(fs) may correspond to at least a maximum optical carrier bandwidth.

In another aspect a disclosed optical transport network is enabled for using modulation frequencies for optical channel monitoring with periodic optical filtering. The optical transport network may include a control system, a transmitter, and an optical channel monitor. In the optical transport network, the control system may be enabled to divide an optical band into bandwidth slices, each of the bandwidth slices having an equal spectral band that is smaller than a minimum optical carrier bandwidth, and to assign, to each of the bandwidth slices, a modulation frequency that is respectively unique to each of the bandwidth slices. In the optical transport network, the control system may further be enabled to, based on a spectral position of a first bandwidth slice, assign a first optical carrier aligned with the spectral position to a first modulation frequency corresponding to the first bandwidth slice. In the optical transport network, the transmitter may be enabled to transmit the first optical carrier over the optical transport network, including modulating the first optical carrier using a first modulation frequency. In the optical transport network, the first optical carrier may be included with a plurality of optical carriers transmitted over the optical transport network. In the optical transport network, the optical channel monitor may be enabled to receive the optical carriers transmitted over at least a portion of the optical transport network, and to scan the optical carriers using a periodic optical filter over a free optical spectral range W_(fs) of the periodic optical filter to generate a filtered signal. In the optical transport network, the free optical spectral range W_(fs) may correspond to K number of optical carriers and K may be greater than or equal to 2. In the optical transport network, the optical channel monitor may further be enabled to demodulate the filtered signal to generate a power spectrum within the free spectral range for the first modulation frequency. In the optical transport network, based on the first bandwidth slice and the power spectrum for the first modulation frequency, the optical channel monitor may still further be enabled to identify a first center optical frequency f_(n) of the first optical carrier received.

In any of the disclosed embodiments of the optical transport network, when a first carrier frequency error for the first center optical frequency is greater than the equal spectral band, the optical channel monitor may calculate a second center optical frequency f_(n′), and, based on f_(n′), calculating a second carrier frequency error. When the second carrier frequency error is less than the equal spectral band, the optical channel monitor may determine that f_(n′) is an actual center optical frequency for the first optical carrier.

In any of the disclosed embodiments of the optical transport network, when a first carrier frequency error for the first center optical frequency is greater than the equal spectral band, the optical channel monitor may calculate a third center optical frequency f_(1″), and, based on f_(n″), calculate a third carrier frequency error. When the third carrier frequency error is less than the equal spectral band, the optical channel monitor may determine that f_(n″) is an actual center optical frequency for the first optical carrier.

In any of the disclosed embodiments of the optical transport network, a carrier frequency error for a carrier frequency f may be calculated using at least one of: an absolute value of a difference between f and a maximum frequency of the bandwidth slice corresponding to the modulation frequency f, and an absolute value of a difference between f and a minimum frequency of the bandwidth slice corresponding to the modulation frequency f.

In any of the disclosed embodiments of the optical transport network, the first optical carrier may be modulated using frequency modulation (FM), and the central alignment feature may be a relative minima in the power spectrum. In any of the disclosed embodiments of the optical transport network, the first optical carrier may be modulated using amplitude modulation (AM), and the central alignment feature may be a relative maxima in the power spectrum.

In any of the disclosed embodiments of the optical transport network, the optical channel monitor demodulating the filtered signal may include using a digital signal processor to detect at least the first modulation frequency.

In any of the disclosed embodiments of the optical transport network, the optical channel monitor demodulating the filtered signal may include, when the first modulation frequency is not detected from the filtered signal, determining that the first optical carrier is not included in the optical carriers received. In the optical transport network, the optical carriers may be subcarriers of a superchannel, while the free optical spectral range W_(fs) may correspond to at least a maximum optical carrier bandwidth.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention and its features and advantages, reference is now made to the following description, taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a block diagram of selected elements of an embodiment of an optical transport network for superchannel subcarrier monitoring;

FIG. 2 shows selected elements of an embodiment of a superchannel power spectrum;

FIG. 3A is a subcarrier power spectrum showing selected elements of an embodiment of FM tone modulation;

FIG. 3B is a subcarrier power spectrum showing selected elements of an embodiment of AM tone modulation;

FIG. 4A is a subcarrier power spectrum showing selected elements of an embodiment of subcarrier monitoring with FM tone modulation;

FIG. 4B is a subcarrier power spectrum showing selected elements of an embodiment of subcarrier monitoring with AM tone modulation;

FIG. 5A is a subcarrier power spectrum showing selected elements of an embodiment of subcarrier monitoring with FM tone modulation;

FIG. 5B is a subcarrier power spectrum showing selected elements of an embodiment of subcarrier monitoring with AM tone modulation;

FIG. 6A is a block diagram of selected elements of an embodiment of modulation and detection for superchannel subcarrier monitoring with FM tone modulation;

FIG. 6B is a block diagram of selected elements of an embodiment of modulation and detection for superchannel subcarrier monitoring with AM tone modulation;

FIG. 7 is a block diagram of selected elements of an embodiment of an optical control plane system for superchannel subcarrier monitoring;

FIG. 8 is a power spectrum and a transmittance spectrum of selected elements of an embodiment of periodic optical filtering to identify tone modulated optical signals;

FIG. 9 is a block diagram of selected elements of an embodiment of tone modulation and detection using a periodic optical filter;

FIG. 10 is a diagram of selected elements of an embodiment of an optical spectrum; and

FIG. 11 is a flow chart of selected elements of an embodiment of a method for using modulation frequencies.

DESCRIPTION OF THE EMBODIMENT(S)

In the following description, details are set forth by way of example to facilitate discussion of the disclosed subject matter. It should be apparent to a person of ordinary skill in the field, however, that the disclosed embodiments are exemplary and not exhaustive of all possible embodiments.

As used herein, a hyphenated form of a reference numeral refers to a specific instance of an element and the un-hyphenated form of the reference numeral refers to the collective or generic element. Thus, for example, widget “72-1” refers to an instance of a widget class, which may be referred to collectively as widgets “72” and any one of which may be referred to generically as a widget “72”.

Referring now to the drawings, FIG. 1 illustrates an example embodiment of optical transport network (OTN) 101, which may represent an optical communication system. Optical transport network 101 included one or more optical fibers 106 to transport one or more optical signals communicated by components of optical transport network 101. The network elements of optical transport network 101, coupled together by fibers 106, may comprise one or more transmitters (Tx) 102, one or more multiplexers (MUX) 104, one or more optical amplifiers 108, one or more optical add/drop multiplexers (OADM) 110, one or more demultiplexers (DEMUX) 105, and one or more receivers (Rx) 112.

Optical transport network 101 may comprise a point-to-point optical network with terminal nodes, a ring optical network, a mesh optical network, or any other suitable optical network or combination of optical networks. Optical transport network 101 may be used in a short-haul metropolitan network, a long-haul inter-city network, or any other suitable network or combination of networks. The capacity of optical transport network 101 may include, for example, 100 Gbit/s, 400 Gbit/s, or 1 Tbit/s. Optical fibers 106 comprise thin strands of glass capable of communicating the signals over long distances with very low loss. Optical fibers 106 may comprise a suitable type of fiber selected from a variety of different fibers for optical transmission. Optical fibers 106 may include any suitable type of fiber, such as a standard Single-Mode Fiber (SMF), Enhanced Large Effective Area Fiber (E-LEAF), or TrueWave® Reduced Slope (TW-RS) fiber.

Optical transport network 101 may include devices to transmit optical signals over optical fibers 106. Information may be transmitted and received through optical transport network 101 by modulation of one or more wavelengths of light to encode the information on the wavelength. In optical networking, a wavelength of light may also be referred to as a “channel” that is included in an optical signal. Each channel may carry a certain amount of information through optical transport network 101.

To increase the information capacity and transport capabilities of optical transport network 101, multiple signals transmitted at multiple channels may be combined into a single wide bandwidth optical signal. The process of communicating information at multiple channels is referred to in optics as wavelength division multiplexing (WDM). Coarse wavelength division multiplexing (CWDM) refers to the multiplexing of wavelengths that are widely spaced having low number of channels, usually greater than 20 nm and less than sixteen wavelengths, and dense wavelength division multiplexing (DWDM) refers to the multiplexing of wavelengths that are closely spaced having large number of channels, usually less than 0.8 nm spacing and greater than forty wavelengths, into a fiber. WDM or other multi-wavelength multiplexing transmission techniques are employed in optical networks to increase the aggregate bandwidth per optical fiber. Without WDM, the bandwidth in optical networks may be limited to the bit-rate of solely one wavelength. With more bandwidth, optical networks are capable of transmitting greater amounts of information. Optical transport network 101 may transmit disparate channels using WDM or some other suitable multi-channel multiplexing technique, and to amplify the multi-channel signal.

Recently, advancements in DWDM enabled combining several optical carriers to create a composite optical signal of a desired capacity. One such example of a multi-carrier optical signal is a superchannel, which is an example of high spectral efficiency (SE) that may attain transmission rates of 100 Gb/s or higher. Thus, in a superchannel, subcarriers are tightly packed and consume less optical spectrum than conventional DWDM. Another distinctive feature of superchannels is that the subcarriers in a superchannel travel as a single entity from the same origin to the same destination, and are not typically added or removed using a conventional OADM while in transmission. In some embodiments, the subcarriers in a superchannel may be added and dropped using a wavelength selective switch (WSS). Techniques for achieving high spectral efficiency (SE) in optical networks may include the use of superchannels modulated using dual-polarization quadrature phase-shift keying (DP-QPSK) for long-haul transmission at data rates of 100 Gb/s or greater. In particular embodiments, Nyquist wavelength-division multiplexing (N-WDM) may be used in a superchannel. In N-WDM, optical pulses having a nearly rectangular spectrum are packed together in the frequency domain with a bandwidth approaching the Baud rate (see also FIG. 2).

Optical transport network 101 may include one or more optical transmitters (Tx) 102 to transmit optical signals through optical transport network 101 in specific wavelengths or channels. Transmitters 102 may comprise a system, apparatus or device to convert an electrical signal into an optical signal and transmit the optical signal. For example, transmitters 102 may each comprise a laser and a modulator to receive electrical signals and modulate the information contained in the electrical signals onto a beam of light produced by the laser at a particular wavelength, and transmit the beam for carrying the signal throughout optical transport network 101.

Multiplexer 104 may be coupled to transmitters 102 and may be a system, apparatus or device to combine the signals transmitted by transmitters 102, e.g., at respective individual wavelengths, into a WDM signal.

Optical amplifiers 108 may amplify the multi-channeled signals within optical transport network 101. Optical amplifiers 108 may be positioned before and after certain lengths of fiber 106, which is referred to as “in-line amplification”. Optical amplifiers 108 may comprise a system, apparatus, or device to amplify optical signals. For example, optical amplifiers 108 may comprise an optical repeater that amplifies the optical signal. This amplification may be performed with opto-electrical or electro-optical conversion. In some embodiments, optical amplifiers 108 may comprise an optical fiber doped with a rare-earth element to form a doped fiber amplification element. When a signal passes through the fiber, external energy may be applied in the form of a pump signal to excite the atoms of the doped portion of the optical fiber, which increases the intensity of the optical signal. As an example, optical amplifiers 108 may comprise an erbium-doped fiber amplifier (EDFA). However, any other suitable amplifier, such as a semiconductor optical amplifier (SOA), may be used.

OADMs 110 may be coupled to optical transport network 101 via fibers 106. OADMs 110 comprise an add/drop module, which may include a system, apparatus or device to add and drop optical signals (i.e., at individual wavelengths) from fibers 106. After passing through an OADM 110, an optical signal may travel along fibers 106 directly to a destination, or the signal may be passed through one or more additional OADMs 110 and optical amplifiers 108 before reaching a destination. In this manner, OADMs 110 may enable connection of different optical transport network topologies together, such as different rings and different linear spans.

In certain embodiments of optical transport network 101, OADM 110 may represent a reconfigurable OADM (ROADM) that is capable of adding or dropping individual or multiple wavelengths of a WDM signal. The individual or multiple wavelengths may be added or dropped in the optical domain, for example, using a wavelength selective switch (WSS) (not shown) that may be included in a ROADM.

Many existing optical networks are operated at 10 gigabit-per-second (Gbps) or 40 Gbps signal rates with 50 gigahertz (GHz) of channel spacing in accordance with International Telecommunications Union (ITU) standard wavelength grids, also known as fixed-grid spacing, which is compatible with conventional implementations of optical add-drop multiplexers (OADMs) and with conventional implementations of demultiplexers 105. However, as data rates increase to 100 Gbps and beyond, the wider spectrum requirements of such higher data rate signals often require increasing channel spacing. In traditional fixed grid networking systems supporting signals of different rates, the entire network system typically must be operated with the coarsest channel spacing (100 GHz, 200 GHz, etc.) that can accommodate the highest rate signals. This may lead to an over-provisioned channel spectrum for lower-rate signals and lower overall spectrum utilization.

Thus, in certain embodiments, optical transport network 101 may employ components compatible with flexible grid optical networking that enables specifying a particular frequency slot per channel. For example, each wavelength channel of a WDM transmission may be allocated using at least one frequency slot. Accordingly, one frequency slot may be assigned to a wavelength channel whose symbol rate is low, while a plurality of frequency slots may be assigned to a wavelength channel whose symbol rate is high. Thus, in optical transport network 101, ROADM 110 may be capable of adding or dropping individual or multiple wavelengths of a WDM, DWDM, or superchannel signal carrying data channels to be added or dropped in the optical domain. In certain embodiments, ROADM 110 may include or be coupled to a wavelength selective switch (WSS).

As shown in FIG. 1, optical transport network 101 may also include one or more demultiplexers 105 at one or more destinations of network 101. Demultiplexer 105 may comprise a system apparatus or device that acts as a demultiplexer by splitting a single composite WDM signal into individual channels at respective wavelengths. For example, optical transport network 101 may transmit and carry a forty (40) channel DWDM signal. Demultiplexer 105 may divide the single, forty channel DWDM signal into forty separate signals according to the forty different channels. It will be understood that different numbers of channels or subcarriers may be transmitted and demultiplexed in optical transport network 101, in various embodiments.

In FIG. 1, optical transport network 101 may also include receivers 112 coupled to demultiplexer 105. Each receiver 112 may receive optical signals transmitted at a particular wavelength or channel, and may process the optical signals to obtain (demodulate) the information (data) that the optical signals contain. Accordingly, network 101 may include at least one receiver 112 for every channel of the network.

Optical networks, such as optical transport network 101 in FIG. 1, may employ modulation techniques to convey information in the optical signals over the optical fibers. Such modulation schemes may include phase-shift keying (PSK), frequency-shift keying (FSK), amplitude-shift keying (ASK), and quadrature amplitude modulation (QAM), among other examples of modulation techniques. In PSK, the information carried by the optical signal may be conveyed by modulating the phase of a reference signal, also known as a carrier wave, or simply, a carrier. The information may be conveyed by modulating the phase of the signal itself using two-level or binary phase-shift keying (BPSK), four-level or quadrature phase-shift keying (QPSK), multi-level phase-shift keying (M-PSK) and differential phase-shift keying (DPSK). In QAM, the information carried by the optical signal may be conveyed by modulating both the amplitude and phase of the carrier wave. PSK may be considered a subset of QAM, wherein the amplitude of the carrier waves is maintained as a constant.

PSK and QAM signals may be represented using a complex plane with real and imaginary axes on a constellation diagram. The points on the constellation diagram representing symbols carrying information may be positioned with uniform angular spacing around the origin of the diagram. The number of symbols to be modulated using PSK and QAM may be increased and thus increase the information that can be carried. The number of signals may be given in multiples of two. As additional symbols are added, they may be arranged in uniform fashion around the origin. PSK signals may include such an arrangement in a circle on the constellation diagram, meaning that PSK signals have constant power for all symbols. QAM signals may have the same angular arrangement as that of PSK signals, but include different amplitude arrangements. QAM signals may have their symbols arranged around multiple circles, meaning that the QAM signals include different power for different symbols. This arrangement may decrease the risk of noise as the symbols are separated by as much distance as possible. A number of symbols “m” may thus be used and denoted “m-PSK” or “m-QAM.”

Examples of PSK and QAM with a different number of symbols can include binary PSK (BPSK or 2-PSK) using two phases at 0° and 180° (or in radians, 0 and π) on the constellation diagram; or quadrature PSK (QPSK, 4-PSK, or 4-QAM) using four phases at 0°, 90°, 180°, and 270° (or in radians, 0, π/2, π, and 3π/2). Phases in such signals may be offset. Each of 2-PSK and 4-PSK signals may be arranged on the constellation diagram. Certain m-PSK signals may also be polarized using techniques such as dual-polarization QPSK (DP-QPSK), wherein separate m-PSK signals are multiplexed by orthogonally polarizing the signals. Also, m-QAM signals may be polarized using techniques such as dual-polarization 16-QAM (DP-16-QAM), wherein separate m-QAM signals are multiplexed by orthogonally polarizing the signals.

Additionally, polarization division multiplexing (PDM) technology may enable achieving a greater bit rate for information transmission. PDM transmission comprises modulating information onto various polarization components of an optical signal associated with a channel. The polarization of an optical signal may refer to the direction of the oscillations of the optical signal. The term “polarization” may generally refer to the path traced out by the tip of the electric field vector at a point in space, which is perpendicular to the propagation direction of the optical signal.

In certain embodiments, optical transport network 101 may transmit a superchannel, in which a plurality of subcarrier signals (or subchannels or channels) are densely packed in a fixed bandwidth band and may be transmitted at very high data rates, such as 400 Gb/s, 1 Tb/s, or higher. Furthermore, the superchannel may be well suited for transmission over very long distances, such as hundreds of kilometers, for example. A typical super-channel may comprise a set of subcarriers that are frequency multiplexed to form a single channel that are transmitted through optical transport network 101 as one entity. The subcarriers within the superchannel may be tightly packed to achieve high spectral efficiency.

In an optical network, such as optical transport network 101 in FIG. 1, it is typical to refer to a management plane, a control plane, and a transport plane (sometimes called the physical layer). A central management host (not shown) may reside in the management plane and may configure and supervise the components of the control plane. The management plane includes ultimate control over all transport plane and control plane entities (e.g., network elements). As an example, the management plane may consist of a central processing center (e.g., the central management host), including one or more processing resources, data storage components, etc. The management plane may be in electrical communication with the elements of the control plane and may also be in electrical communication with one or more network elements of the transport plane. The management plane may perform management functions for an overall system and provide coordination between network elements, the control plane, and the transport plane. As examples, the management plane may include an element management system (EMS) which handles one or more network elements from the perspective of the elements, a network management system (NMS) which handles many devices from the perspective of the network, or an operational support system (OSS) which handles network-wide operations.

Modifications, additions or omissions may be made to optical transport network 101 without departing from the scope of the disclosure. For example, optical transport network 101 may include more or fewer elements than those depicted in FIG. 1. Also, as mentioned above, although depicted as a point-to-point network, optical transport network 101 may comprise any suitable network topology for transmitting optical signals such as a ring, a mesh, or a hierarchical network topology.

In operation, optical transport network 101 may be used to transmit a superchannel, in which a plurality of subcarrier signals are densely packed in a fixed bandwidth band and may be transmitted at very high data rates, such as 400 Gb/s, 1 Tb/s, or higher. Furthermore, a superchannel may be well suited for transmission over very long distances, such as hundreds of kilometers, for example. A typical superchannel may comprise a set of subcarriers that are frequency multiplexed to form a single channel and which are transmitted through the entire network as one entity. The subcarriers within the superchannel are tightly packed to achieve high spectral efficiency. In order to minimize linear crosstalk between neighboring subcarriers in the superchannel, Nyquist filtering may be applied at the transmitter side to shape the subcarrier frequency bands (see also FIG. 2).

As noted above, optical superchannels may represent a promising solution for transmission of signals at 400 Gb/s and 1 Tb/s data rate per channel, and beyond to higher data rates. However, the optical power transmitted by each subcarrier in a superchannel may fluctuate, which is undesirable. Various transmission experiments with superchannels have revealed that each subcarrier within a superchannel may experience different amounts of linear and non-linear interactions with neighboring subcarriers, resulting in different received optical signal-to-noise ratio (OSNR) penalties. For example, center subcarriers may suffer from larger non-linear interaction compared to edge subcarriers. Additionally, passband narrowing (PBN), which may occur when a subcarrier is transmitted through one or more ROADM nodes, may introduce power variation on the edge subcarriers. Furthermore, a laser source used at transmitter 102 may become detuned and may contribute to power or spectral fluctuation of a subcarrier wavelength. Also, the power across a superchannel may vary when a subcarrier is lost.

Typically, an optical channel monitor (OCM) is used to measure OSNR of a superchannel. However, it may be difficult to detect each individual subcarrier with an OCM. For example, it may be difficult to determine a center frequency of each subcarrier using an OCM, because of the tight spectral packing of the subcarriers in a superchannel, particularly when Nyquist filtering has been applied. In addition, it may be difficult to conclusively identify whether a subcarrier has been lost or whether a spectral fluctuation in wavelength has occurred or to distinguish between these two cases.

As will be described in further detail, various techniques for power and spectral monitoring of individual subcarriers in a superchannel are disclosed. The superchannel subcarrier monitoring techniques disclosed herein may enable precise and accurate detection of individual subcarriers in the spectral domain. The superchannel subcarrier monitoring techniques disclosed herein may enable direct measurement of edge frequencies for each subcarrier in a superchannel. The superchannel subcarrier monitoring techniques disclosed herein may enable determination of a center frequency for each subcarrier in a superchannel. The superchannel subcarrier monitoring techniques disclosed herein may enable monitoring of optical power at the center frequency for each subcarrier in a superchannel. The superchannel subcarrier monitoring techniques disclosed herein may enable monitoring of spectral narrowing for each subcarrier in a superchannel, such as caused by passband narrowing.

In one technique, each subcarrier in a superchannel may be assigned a frequency modulated (FM) tone having a unique frequency, such as in a radio frequency (RF) range. The unique FM tone frequency may then be modulated onto each subcarrier using an optical transmitter. The FM tone may be superimposed with the optical signal representing a data payload transmitted by each subcarrier. Then, an optical subcarrier monitor may be used to precisely and accurately detect the FM tone, thereby enabling monitoring of each subcarrier in the superchannel, as will be explained in further detail.

In another technique, each subcarrier in a superchannel may be assigned an amplitude modulated (AM) tone having a unique frequency, such as in a kilohertz (kHz) frequency range or greater. The unique AM tone frequency may then be modulated onto each subcarrier using an optical transmitter. The AM tone may be superimposed with the optical signal representing a data payload transmitted by each subcarrier. Then, an optical subcarrier monitor may be used to precisely and accurately detect the AM tone, thereby enabling monitoring of each subcarrier in the superchannel, as will be explained in further detail.

In some embodiments, various combinations of FM tones and AM tones may be used for superchannel subcarrier monitoring.

Referring to FIG. 2, selected elements of an embodiment of a superchannel is shown as superchannel power spectrum 200, which depicts five (5) subcarriers. While the data used for superchannel power spectrum 200 are not actual measured values, the illustrated power spectrum may be characteristic of an actual superchannel. In superchannel power spectrum 200, the subcarriers may each be modulated with 200 GB/s DP-16-QAM signals. Furthermore, each subcarrier band has been subject to electrical Nyquist pulse shaping in the transmitter using a root raised cosine method using a roll-off factor of 0.15. As shown in FIG. 2, B_(SC) represents the fixed superchannel transmission band, while Δf represents the subcarrier frequency spacing. In certain embodiments, the subcarrier frequency spacing Δf may be 35 GHz and may be uniform between each center frequency f₁, f₂, f₃, f₄, and f₅, respectively corresponding to the subcarrier bands. The subcarrier frequency spacing Δf may be selected to be wide enough to prevent any significant linear crosstalk between adjacent subcarriers. The optical signal of each subcarrier may be multiplexed using an optical coupler to form the single superchannel in the fixed transmission band B_(SC) having an aggregate data rate of 1 Tb/s, for example. It is noted that different values for the fixed superchannel transmission band, B_(SC), the subcarrier frequency spacing Δf, and the overall aggregate data rate may result in superchannel power spectrum 200. Also shown in FIG. 2 is constant power level, P_(SC), that is a power level for the superchannel that is substantially similar or equal for each of the 5 subcarrier bands, such that P_(SC), may correspond to an average power level for each of the subcarrier bands.

In typical DWDM networks, it is known that system performance may depend on an allocation of each wavelength channel on the wavelength grid, such that a longer wavelength channel may suffer from smaller non-linear impairments compared to a shorter wavelength channel. In case of superchannel-based WDM systems, in addition to the wavelength dependency of the subcarrier error rate across the transmission band, B_(SC), a dependency of individual subcarrier error rate (or OSNR at the receiver) on spectral allocation of the subcarrier within the superchannel has now been observed in the form of non-linear impairments (such as cross-talk). Linear cross-talk may be observed between two adjacent subcarrier bands (inter-subcarrier) and may depend on a degree or extent of overlap in the frequency domain of the adjacent subcarrier bands. The use of Nyquist pulse shaping, as shown in FIG. 2, may represent an effective means for maintaining a minimum level of linear cross-talk between adjacent subcarriers, at least in part due to the nearly vertical edges of the Nyquist-shaped subcarrier bands (spectral pulses) that do not substantially overlap each other in the frequency domain. Non-linear cross-talk may also be observed and may arise from non-linear interactions during fiber transmission. The non-linear interactions may include phenomena such as cross-phase modulation (XPM), self-phase modulation (SPM), and four-wave mixing, among others. Cross-phase modulation may occur when phase information, amplitude information, or both from one channel is modulated to an adjacent channel in the superchannel. Self-phase modulation may arise when a variation in the refractive index (or a dependency of the refractive index on intensity) results in a phase shift within each subcarrier. In four-wave mixing, three wavelengths may interact to create a fourth wavelength that may coincide with a wavelength of a subcarrier, and may lead to undesirable variations in peak power or other types of signal distortion on the affected subcarrier. Furthermore, non-linear cross-talk may comprise inter-subcarrier components. Since non-linear interactions occur during fiber transmission and may not depend on a degree of overlap of the subcarrier frequency bands, Nyquist pulse shaping may be ineffective in resolving problems with non-linear cross-talk in a superchannel.

As will be described in further detail herein, at least some of the subcarrier bands depicted in simulated frequency spectrum 200 may be modulated with a unique FM tone for superchannel subcarrier monitoring. As will also be described in further detail herein, at least some of the subcarrier bands depicted in simulated frequency spectrum 200 may be modulated with an AM tone for superchannel subcarrier monitoring. In some embodiments, certain combinations of modulation techniques using FM tones and AM tones may be applied to subcarriers used for superchannel subcarrier monitoring.

Referring now to FIG. 3A, a subcarrier power spectrum 300 showing selected elements of an embodiment of FM tone modulation is illustrated. In subcarrier power spectrum 300, a single optical subcarrier is shown as subcarrier band 302. As shown in FIG. 3A, subcarrier band 302 is depicted in simplified form for descriptive clarity, yet may still correspond to the subcarrier bands depicted in superchannel power spectrum 200 (see FIG. 2). Although a single instance of subcarrier band 302 is depicted in FIG. 3A for descriptive purposes, it will be understood that subcarrier band 302 may be representative of each of a plurality of subcarrier bands included in a superchannel (see also FIG. 5A). Thus, subcarrier power spectrum 300 is a generalized representation and is not drawn to scale.

In subcarrier power spectrum 300, subcarrier band 302 has center frequency f_(C) and spans from initial frequency f₀ to terminal frequency f_(T). Accordingly, a passband for subcarrier band 302 may be given by (f_(T)−f₀). Also shown in FIG. 3A is power level, P_(C), representing an optical power level indicative of subcarrier band 302 at center frequency f_(C). As shown, subcarrier band 302 is frequency modulated using a frequency range ν_(ΔFM), which may represent a modulation range about a unique frequency ν_(FM) for subcarrier band 302 within a superchannel, or within an optical transport network transmitting a plurality of superchannels. Accordingly, a modulation index using FM modulation is given by ν_(ΔFM)/ν_(FM). In various embodiments, ν_(FM) (or values within ν_(ΔFM)) may be smaller than an optical transmission frequency (or baud rate) of a data stream of carrier data that is also modulated onto subcarrier band 302, for example using any of a variety of modulation techniques described above with respect to FIG. 1. In various embodiments, ν_(FM) and ν_(ΔFM) are in the radio frequency (RF) range of the spectrum. For example, ν_(FM) may be less than about 300 MHz. It is noted that an FM modulation amplitude is represented schematically by ν_(ΔFM) and is not illustrated to scale in FIG. 3A, but may be chosen as desired and suitable for a given application or embodiment of superchannel subcarrier monitoring using FM tone modulation. Thus, upon modulation with carrier data and with FM tone using frequency range ν_(ΔFM), subcarrier band 302 may be transmitted at least over certain portions of optical transport network 101 (see FIG. 1), along with other subcarrier bands as a superchannel.

Then, at various desired locations within optical transport network 101, optical subcarrier monitoring may be performed to determine various characteristics of subcarrier band 302 during or after transmission, as will be described in further detail in FIG. 4A. For the purpose of optical subcarrier monitoring, a tunable bandpass filter (TBPF), represented by TBPF band 304, may be tuned over an optical frequency range, such as over a scanning range of f₀ to f_(T), and used to scan subcarrier band 302, among other subcarrier bands. As shown, the scanning may be performed in direction 320. In other embodiments, other directions or methods of scanning subcarrier band 320 may be implemented. As TBPF band 304 is used to scan subcarrier band 302 with FM tone modulation, the falling or rising edge of TBPF band 304 may act as a frequency discriminator, such that an output of the tunable bandpass filter shows a conversion of the FM tone at ν_(FM) to a power variation when TBPF band 304 passes through a rising or falling edge of subcarrier band 302, which is described below with respect to FIG. 4A.

FIG. 4A shows selected elements of an embodiment of subcarrier monitoring 400 with FM tone modulation, where power variation 402 results from the scanning of subcarrier band 302 using TBPF band 304, as described above with respect to FIG. 3A. In FIG. 4A, the horizontal axis is the scanning frequency of the TBPF that represents a center of TBPF band 304. From the scanning of the TBPF, power variation 402 is generated as a power spectrum indicating a bandpass filtered RF power at the modulation frequency ν_(FM). For example, in some implementations of subcarrier monitoring, an RF bandpass filter centered at ν_(FM) may be used to obtain power variation 402. Accordingly, power variation 402 is proportional to a convolution of the optical TBPF and the absolute value of a differential of modulated subcarrier band 302 in FIG. 3A. The optical TBPF serves as a frequency discriminator for the demodulation of the modulation frequency ν_(FM). As shown, power variation 402 appears as a dual-peak shaped power spectrum signal in which a first spectral peak is approximately aligned with initial frequency f₀′ and a second spectral peak is approximately aligned with terminal frequency f_(T)′. The passband of the optical signal given by (f_(T)−f₀) may be calibrated to observed values of f₀′ and f_(T)′ and used for subsequent determinations of the passband. Between the first peak at f₀′ and the second peak at f_(T)′, a spectral valley appears in power variation 402 at that is approximately aligned with center frequency f_(C). In this manner, power variation 402 may be used to directly measure spectral frequencies and the passband of subcarrier band 302 in a manner that is precise and accurate. Furthermore, power level P_(FM) may be directly measured from power variation 402 as an amplitude of the first peak at f₀, the second peak at f_(T), or combinations thereof. Power level P_(FM) may represent a maximum power level at ν_(FM) and may be proportional, or indicative, of P_(C) for subcarrier band 302. In this manner, optical power P_(C) for subcarrier band 302 may be determined using power variation 402.

Referring now to FIG. 3B, a subcarrier power spectrum 301 showing selected elements of an embodiment of AM tone modulation is illustrated. In subcarrier power spectrum 301, a single optical subcarrier is shown as subcarrier band 302. As shown in FIG. 3B, subcarrier band 302 is depicted in simplified form for descriptive clarity, yet may still correspond to the subcarrier bands depicted in superchannel power spectrum 200 (see FIG. 2). Although a single instance of subcarrier band 302 is depicted in FIG. 3B for descriptive purposes, it will be understood that subcarrier band 302 may be representative of each of a plurality of subcarrier bands included in a superchannel (see also FIG. 5B). Thus, subcarrier power spectrum 301 is a generalized representation and is not drawn to scale.

In subcarrier power spectrum 301, subcarrier band 302 has center frequency f_(C) and spans from initial frequency f₀ to terminal frequency f_(T). Accordingly, a passband for subcarrier band 302 may be given by (f_(T)−f₀). Also shown in FIG. 3B is power level, P_(C), representing an optical power level indicative of subcarrier band 302 at center frequency f_(C). As shown, subcarrier band 302 is amplitude modulated using a frequency ν_(AM) with a peak-peak power modulation amplitude given by P_(ΔAM). In various embodiments, ν_(AM) may be substantially smaller than an optical transmission frequency (or baud rate) of a data stream of carrier data that is also modulated onto subcarrier band 302, for example using any of a variety of modulation techniques described above with respect to FIG. 1. In various embodiments, ν_(AM) is in the RF range of the spectrum. For example, ν_(AM) may be less than about 300 MHz. It is noted that P_(ΔAM) is represented schematically for ν_(AM) and is not illustrated to scale in FIG. 3B, but may be chosen as desired and suitable for a given application or embodiment of superchannel subcarrier monitoring using AM tone modulation. For example, P_(ΔAM) may be about 5% of P_(c) in given embodiments. Furthermore, a phase of the AM tone at frequency ν_(AM) may be chosen for subcarrier band 302, as will be described in further detail with respect to FIG. 6B. Thus, upon modulation with carrier data and with the AM tone at frequency ν_(AM), subcarrier band 302 may be transmitted at least over certain portions of optical transport network 101 (see FIG. 1), along with other subcarrier bands as a superchannel.

Then, at various desired locations within optical transport network 101, optical subcarrier monitoring may be performed to determine various characteristics of subcarrier band 302 during or after transmission, as will be described in further detail in FIG. 4B. For the purpose of optical subcarrier monitoring, a tunable bandpass filter (TBPF), represented by TBPF band 304, may be tuned over an optical frequency range, such as over a scanning range of f₀ to f_(T), and used to scan subcarrier band 302, among other subcarrier bands. As shown, the scanning may be performed in direction 320. In other embodiments, other directions or methods of scanning subcarrier band 320 may be implemented. As TBPF band 304 is used to scan subcarrier band 302 with AM tone modulation, a power variation profile that is proportional to a convolution of the optical TBPF and the optical spectral profile of subcarrier band 302 in FIG. 3B is generated. The resulting power variation of the AM tone will follow the general optical spectral profile of subcarrier band 302, as TBPF band 304 passes through a rising or falling edge of subcarrier band 302, which is described below with respect to FIG. 4B.

FIG. 4B shows selected elements of an embodiment of subcarrier monitoring 401 with AM tone modulation, where power variation 404 results from the scanning of subcarrier band 302 using TBPF band 304, as described above with respect to FIG. 3B. In FIG. 4B, the horizontal axis is the scanning frequency of the tunable bandpass filter that represents a center of TBPF band 304. From the scanning of the tunable bandpass filter, power variation 404 is generated as a power spectrum indicating a bandpass filtered RF power at the modulation frequency ν_(AM). As shown, power variation 404 appears as a power spectrum band that is similarly shaped as subcarrier band 302. Specifically, power variation 404 is centered at center frequency f_(C)′ and exhibits a bandwidth 406 that is proportional to, or indicative of, passband (f_(T)−f₀). The measured center frequency f_(C)′ may be aligned with, or correlated with, center frequence f_(c). Furthermore, power level P_(AM) may be directly measured from power variation 404 as an amplitude at f_(C)′. Power level P_(AM) may represent a maximum power level at ν_(AM) and may be proportional, or indicative, of P_(C) for subcarrier band 302. In this manner, optical power P_(C) for subcarrier band 302 may be determined using power variation 404.

Referring now to FIG. 5A, an embodiment of subcarrier monitoring 500 with FM tone modulation is illustrated for a superchannel corresponding to superchannel power spectrum 200 in FIG. 2. In subcarrier monitoring 500, power variations 502 result from the scanning of five (5) subcarrier bands at center frequencies, f₁, f₂, f₃, f₄, and f₅, respectively. Power variation 502-1 is for a first subcarrier band having center frequency f₁ and modulated with FM tone having frequency ν_(FM1); power variation 502-2 is for a second subcarrier band having center frequency f₂ and modulated with FM tone having frequency ν_(FM2), power variation 502-3 is for a third subcarrier band having center frequency f₃ and modulated with FM tone having frequency ν_(FM3); power variation 502-4 is for a fourth subcarrier band having center frequency f₄ and modulated with FM tone having frequency ν_(FM4); and power variation 502-5 is for a fifth subcarrier band having center frequency f₅ and modulated with FM tone having frequency ν_(FM5). It is noted that FM tone frequencies ν_(FM1), ν_(FM2), ν_(FM3), ν_(FM4), and ν_(FM5) may be unique frequencies within the superchannel or within the optical transport network. As in FIG. 4A, power variations 502 appear as dual-peak shaped power signals for each respective subcarrier band. Because each instance of power variation 502 is specific to a unique frequency ν_(FMn), each power variation 502 may be individually resolved to detect, uniquely identify, and monitor a specific subcarrier band using FM tone modulation, as described herein.

Referring now to FIG. 5B, an embodiment of subcarrier monitoring 501 with AM tone modulation is illustrated for a superchannel corresponding to superchannel power spectrum 200 in FIG. 2. In subcarrier monitoring 501, power variations 504 result from the scanning of five (5) subcarrier bands at center frequencies, f₁, f₂, f₃, f₄, and f₅, respectively. Power variation 504-1 is for a first subcarrier band having center frequency f₁ and modulated with AM tone having frequency ν_(AM1); power variation 504-2 is for a second subcarrier band having center frequency f₂ and modulated with AM tone having frequency ν_(AM2); power variation 504-3 is for a third subcarrier band having center frequency f₃ and modulated with AM tone having frequency ν_(AM3); power variation 504-4 is for a fourth subcarrier band having center frequency f₄ and modulated with AM tone having frequency ν_(AM4); and power variation 504-5 is for a fifth subcarrier band having center frequency f₅ and modulated with AM tone having frequency ν_(AM5). It is noted that AM tone frequencies ν_(AM1), ν_(AM2), ν_(AM3), ν_(AM4), and ν_(AM5) may be individually associated with a phase shift and certain pairs of the AM tone frequencies may be common frequencies (see also FIG. 6B). As in FIG. 4B, power variations 504 appear as band shaped power signals for each respective subcarrier band. Because each instance of power variation 504 is specific to a subcarrier band, each power variation 504 may be individually resolved to detect, uniquely identify, and monitor a specific subcarrier band using AM tone modulation, as described herein.

Referring now to FIG. 6A, a block diagram of selected elements of an embodiment of FM tone modulation and detection 600 for superchannel subcarrier monitoring is illustrated. Although FM tone modulation and detection 600 depicts a single subcarrier, it will be understood that multiple subcarriers may be modulated with FM tones and detected for a given superchannel. It is noted that functionality described with respect to FIGS. 6A and 6B, representing FM tone modulation and AM tone modulation, respectively, may be combined or may be implemented with common components, in various embodiments. At least certain portions of optical subcarrier monitor 620 may be included in an optical channel monitor (OCM).

In FM tone modulation and detection 600, FM tone modulation is performed using optical transmitter 606-1, which may be an embodiment of transmitter 102 in FIG. 1. Thus, optical transmitter 606-1 may receive and modulate data onto the optical subcarrier according to various types of modulation and carrier techniques described herein. Additionally, optical transmitter 606-1 may modulate FM tone 604 onto the optical subcarrier, which is combined with other subcarriers into the superchannel and transmitted over optical transport network 602. As noted, FM tone 604 may be unique to a given optical subcarrier within the superchannel or within optical transport network 602, which may represent at least certain portions of optical transport network 101 in FIG. 1.

At some location within optical transport network 602, optical subcarrier monitor 620-1 may be employed to receive the superchannel and to detect and monitor individual optical subcarriers. Accordingly, optical subcarrier monitor 620-1 includes tunable bandpass filter (TBPF) 608 for spectral scanning using TBPF band 304, as described above with respect to FIG. 3A. An output of tunable bandpass filter 608 is received by optical power sensor 610, which may represent a photodiode or a similar opto-electronic element, which outputs an electrical signal to analog-to-digital converter (ADC) 612. ADC 612 may digitize the electrical signal and send digital signals or data to FM tone demodulator 614. FM tone demodulator 614 may include various combinations of hardware and software, such as a digital signal processor (DSP) and associated executable instructions, for demodulating FM tone 604 and generating power variation 402 (see FIG. 4A) indicative of the optical subcarrier. It is noted that optical subcarrier monitor 620-1 may communicate with optical transmitter 606-1 using a network control system, such as control system 700 in FIG. 7, for example, to receive modulation information, such as a frequency of FM tone 604. Because the frequency of FM tone 604 is unique, detection of the frequency by optical subcarrier monitor 620-1 may uniquely identify the optical subcarrier. Conversely, when the unique frequency of FM tone 604 that is assigned to an optical subcarrier is not detected by optical subcarrier monitor 620-1, then a determination may be made that the optical subcarrier is missing from the superchannel.

Referring now to FIG. 6B, a block diagram of selected elements of an embodiment of AM tone modulation and detection 601 for superchannel subcarrier monitoring is illustrated. Although AM tone modulation and detection 601 depicts a single subcarrier, it will be understood that multiple subcarriers may be modulated with AM tones and detected for a given superchannel. It is noted that functionality described with respect to FIGS. 6A and 6B, representing FM tone modulation and AM tone modulation, respectively, may be combined or may be implemented with common components, in various embodiments. At least certain portions of optical subcarrier monitor 620 may be included in an optical channel monitor (OCM).

In AM tone modulation and detection 601, AM tone modulation is performed using optical transmitter 606-2, which may be an embodiment of transmitter 102 in FIG. 1. Thus, optical transmitter 606-2 may receive and modulate carrier data onto the optical subcarrier according to various types of modulation and carrier techniques described herein. Additionally, optical transmitter 606-2 may modulate AM tone 603 onto the optical subcarrier, which is combined with other subcarriers into the superchannel and transmitted over optical transport network 602, which may represent at least certain portions of optical transport network 101 in FIG. 1. Additionally, a phase delay 605 may be applied to AM tone 603 prior to modulation at optical transmitter 606-2. It is noted that in some embodiments, phase delay 605 may be integrated within AM tone 603 or optical transmitter 606-2.

At some location within optical transport network 602, optical subcarrier monitor 620-2 may be employed to receive the superchannel and to detect and monitor individual optical subcarriers. Accordingly, optical subcarrier monitor 620-2 includes tunable bandpass filter (TBPF) 608 for spectral scanning using TBPF band 304, as described above with respect to FIG. 3B. An output of tunable bandpass filter 608 is received by optical power sensor 610, which may represent a photodiode or a similar opto-electronic element, which outputs an electrical signal to analog-to-digital converter (ADC) 612. ADC 612 may digitize the electrical signal and send digital signals or data to AM tone demodulator 616. AM tone demodulator 616 may include various combinations of hardware and software, such as a digital signal processor (DSP) and associated executable instructions, for demodulating AM tone 603 and phase delay 605 and generating power variation 404 (see FIG. 4B) indicative of the optical subcarrier. It is noted that optical subcarrier monitor 620-2 may communicate with optical transmitter 606-2 using a network control system, such as control system 700 in FIG. 7, for example, to receive modulation information, such as a frequency of AM tone 603 and phase delay 605. Because the combination of frequency and phase of AM tone 603 is unique, detection of the frequency by optical subcarrier monitor 620-2 may uniquely identify the optical subcarrier. Conversely, when a unique combination of frequency and phase of AM tone 603 that is assigned to an optical subcarrier is not detected by optical subcarrier monitor 620-2, then a determination may be made that the optical subcarrier is missing from the superchannel.

Referring now to FIG. 7, a block diagram of selected elements of an embodiment of control system 700 for implementing control plane functionality in optical networks, such as, for example, in optical transport network 101 (see FIG. 1), is illustrated. A control plane may include functionality for network intelligence and control and may comprise applications that support the ability to establish network services, including applications or modules for discovery, routing, path computation, and signaling, as will be described in further detail. The control plane applications executed by control system 700 may work together to automatically establish services within the optical network. Discovery module 712 may discover local links connecting to neighbors. Routing module 710 may broadcast local link information to optical network nodes while populating database 704. When a request for service from the optical network is received, path computation engine 702 may be called to compute a network path using database 704. This network path may then be provided to signaling module 706 to establish the requested service.

As shown in FIG. 7, control system 700 includes processor 708 and memory media 720, which may store executable instructions (i.e., executable code) that may be executable by processor 708, which has access to memory media 720. Processor 708 may execute instructions that cause control system 700 to perform the functions and operations described herein. For the purposes of this disclosure, memory media 720 may include non-transitory computer-readable media that stores data and instructions for at least a period of time. Memory media 720 may comprise persistent and volatile media, fixed and removable media, and magnetic and semiconductor media. Memory media 720 may include, without limitation, storage media such as a direct access storage device (e.g., a hard disk drive or floppy disk), a sequential access storage device (e.g., a tape disk drive), compact disk (CD), random access memory (RAM), read-only memory (ROM), CD-ROM, digital versatile disc (DVD), electrically erasable programmable read-only memory (EEPROM), and flash memory; non-transitory media, or various combinations of the foregoing. Memory media 720 is operable to store instructions, data, or both. Memory media 720 as shown includes sets or sequences of instructions that may represent executable computer programs, namely, path computation engine 702, signaling module 706, discovery module 712, and routing module 710.

Also shown included with control system 700 in FIG. 7 is network interface 714, which may be a suitable system, apparatus, or device operable to serve as an interface between processor 708 and network 730. Network interface 714 may enable control system 700 to communicate over network 730 using a suitable transmission protocol or standard. In some embodiments, network interface 714 may be communicatively coupled via network 730 to a network storage resource. In some embodiments, network 730 represents at least certain portions of optical transport network 101. Network 730 may also include certain portions of a network using galvanic or electronic media. In certain embodiments, network 730 may include at least certain portions of a public network, such as the Internet. Network 730 may be implemented using hardware, software, or various combinations thereof.

In certain embodiments, control system 700 may be configured to interface with a person (a user) and receive data about the optical signal transmission path. For example, control system 700 may also include or may be coupled to one or more input devices and output devices to facilitate receiving data about the optical signal transmission path from the user and to output results to the user. The one or more input or output devices (not shown) may include, but are not limited to, a keyboard, a mouse, a touchpad, a microphone, a display, a touchscreen display, an audio speaker, or the like. Alternately or additionally, control system 700 may be configured to receive data about the optical signal transmission path from a device such as another computing device or a network element, for example via network 730.

As shown in FIG. 7, in some embodiments, discovery module 712 may be configured to receive data concerning an optical signal transmission path in an optical network and may be responsible for discovery of neighbors and links between neighbors. In other words, discovery module 712 may send discovery messages according to a discovery protocol, and may receive data about the optical signal transmission path. In some embodiments, discovery module 712 may determine features, such as, but not limited to: fiber type, fiber length, number and type of components, data rate, modulation format of the data, input power of the optical signal, number of signal carrying wavelengths (i.e., channels), channel spacing, traffic demand, and network topology, among others.

As shown in FIG. 7, routing module 710 may be responsible for propagating link connectivity information to various nodes within an optical network, such as optical transport network 101. In particular embodiments, routing module 710 may populate database 704 with resource information to support traffic engineering, which may include link bandwidth availability. Accordingly, database 704 may be populated by routing module 710 with information usable to determine a network topology of an optical network.

Path computation engine 702 may be configured to use the information provided by routing module 710 to database 704 to determine transmission characteristics of the optical signal transmission path. The transmission characteristics of the optical signal transmission path may provide insight on how transmission degradation factors, such as chromatic dispersion (CD), nonlinear (NL) effects, polarization effects, such as polarization mode dispersion (PMD) and polarization dependent loss (PDL), and amplified spontaneous emission (ASE), among others, may affect optical signals within the optical signal transmission path. To determine the transmission characteristics of the optical signal transmission path, path computation engine 702 may consider the interplay between the transmission degradation factors. In various embodiments, path computation engine 702 may generate values for specific transmission degradation factors. Path computation engine 702 may further store data describing the optical signal transmission path in database 704.

In FIG. 7, signaling module 706 may provide functionality associated with setting up, modifying, and tearing down end-to-end networks services in an optical network, such as optical transport network 101. For example, when an ingress node in the optical network receives a service request, control system 100 may employ signaling module 706 to request a network path from path computation engine 702 that may be optimized according to different criteria, such as bandwidth, cost, etc. When the desired network path is identified, signaling module 706 may then communicate with respective nodes along the network path to establish the requested network services. In different embodiments, signaling module 706 may employ a signaling protocol to propagate subsequent communication to and from nodes along the network path.

In operation of control system 700, optical subcarrier monitor 620 described previously with respect to FIGS. 6A and 6B may receive information about modulation of FM tones or AM tones for a particular subcarrier or all subcarriers in a superchannel. For example, when superchannel subcarrier monitoring using FM tones is implemented at an optical transmitter, as described herein, optical subcarrier monitor 620 may receive unique frequency information from control system 700 for each respective subcarrier that has been modulated with a respectively unique FM tone. When superchannel subcarrier monitoring using AM tones is implemented at an optical transmitter, as described herein, optical subcarrier monitor 620 may receive frequency information and phase information from control system 700 for each respective subcarrier that has been modulated using an AM tone. Optical subcarrier monitor 620 may use the received information for tuning tunable bandpass filter 608 and for demodulating the optical subcarrier, as described above.

In the preceding example embodiments, spectral scanning of an optical signal is described using a tunable bandpass filter, such as tunable bandpass filter 608 in FIGS. 6A and 6B. In certain instances, such as where large numbers of optical channels are present, spectral scanning using a tunable bandpass filter may be associated with relatively long scanning operations or scanning times, which may be undesirable, particularly when performed repeatedly. As will now be described in further detail, spectral scanning of an optical signal may be performed using a periodic optical filter, which are also referred to as comb filters. The use of a periodic optical filter may serve to limit a spectral scanning range of the optical signal, while still enabling spectral scanning of the entire optical signal, by utilizing the periodic nature of the passbands of a periodic optical filter.

Referring now to FIG. 8, periodic optical filtering to identify tone modulated optical signals is illustrated using power spectrum 800 and transmittance spectrum 801. As described with respect to FIG. 8, periodic optical filtering may be performed on a DWDM optical signal in a fixed-grid optical network with uniform bandwidth slices allocated to each optical carrier. It will be understood that in different embodiments, periodic optical filtering may be used with other types of optical signals, such as flex-grid signals and superchannels.

In FIG. 8, power spectrum 800 depicts power versus frequency for a number of optical channels, given by N number of optical channels, that form an optical signal, which are designated by a center frequency for each optical channel f_(n), where n has values of 1 to N. In various embodiments, optical spectrum 800 may depict a DWDM optical signal. The optical signal shown in optical spectrum 800 is also associated with N number of tone modulation frequencies, shown as ν_(n), that are respectively assigned to each optical channel. As shown and described below with respect to FIG. 8, it will be assumed that the frequencies ν_(n) are each unique frequencies used for FM or AM modulation of each of the N optical channels in the optical signal.

In FIG. 8, power spectrum 800 is shown in spectral alignment with transmittance spectrum 801 along the frequency axis. Transmittance spectrum 801 includes periodic passbands 810 that represent the filter functionality of a periodic optical filter. The frequency range between individual passbands 810 is defined as the free spectral range W_(fs) for the periodic optical filter. The free spectral range W_(fs) may be selected to accommodate an integer number of optical channels, which is designated as K number of optical channels. In the exemplary embodiment shown in FIG. 8, K=3 and three (3) optical channels are spaced to fit into the free spectral range W_(fs). In different embodiments, K may be selected to have integer values greater than or equal to 2. In transmittance spectrum 801, first passband 810-1 is initially centered at frequency W_(c1), which corresponds to a start frequency for the optical signal in power spectrum 800. Subsequent passbands 810 to first passband 810-1, given as W_(c2), W_(c3), and so on, are spaced apart from each other according to free spectral range W_(fs).

As scanning is performed using the periodic optical filter, passbands 810 may be scanned in a direction 820 over the free spectral range W_(fs). Because the free spectral range W_(fs) is substantially smaller than a frequency range of the optical signal, scanning the optical signal using the periodic optical filter may be substantially faster than other methods, such as using the tunable bandpass filter described previously. As the periodic optical filter is scanned over free spectral range W_(fs), power spectra corresponding to each respective frequency ν_(n) may be recorded, as described previously for FM modulation (FIG. 4A) and AM modulation (FIG. 4B). Furthermore, the power spectra for frequencies ν_(n) will be periodic with respect to direction 820. As the FM or AM power spectra are generated for each frequency ν_(n), an optical frequency shift ΔW_(n) with respect to W_(fs) will result from a spectral location at which a central alignment feature in each respective power spectrum for a given frequency ν_(n) is detected. As described previously, when FM modulation is used, the power spectra for frequency ν_(n) will be shaped in the form of power variation 402 (see FIG. 4A), and the central alignment feature will be a relative minima. As described previously, when AM modulation is used, the power spectra for frequency ν_(n) will be shaped in the form of power variation 404 (see FIG. 4B), and the central alignment feature will be a relative maxima. The optical frequency shift ΔW_(n), as generated with respect to W_(c1), is shown for the first three optical channels as ΔW₁, ΔW₂, and ΔW₃.

Because optical frequency shift ΔW_(n) is determined with respect to free spectral range W_(fs) after periodic optical filtering, the center optical frequency f_(n) for an optical channel n may be calculated using Equation 1. f _(n) =m*W _(fs) +W _(c1) +ΔW _(n)  Equation (1) In Equation 1, m=floor ((n−1)/K), W_(c1) is an initial scanning frequency of a first periodic passband of the periodic comb filter, and ΔW_(n) is an optical frequency shift with respect to W_(fs) at which a central alignment feature in the power spectrum for frequency ν_(n) is detected.

It is noted that additional criteria may be applied for the free spectral range W_(fs), in some embodiments. For example, in a flex-grid optical network (not shown in FIG. 8), where certain optical carriers may be allocated a larger or a smaller bandwidth slice than other optical carriers, a value of the free spectral range W_(fs) may be selected based on a maximum bandwidth slice (see also FIG. 10). For example, the free spectral range W_(fs) may be specified to be at least the maximum bandwidth slice for any optical carrier in a flex-grid optical network.

Referring now to FIG. 9, a block diagram of selected elements of an embodiment of tone modulation and detection 900 using a periodic optical filter is illustrated. Although tone modulation and detection 900 depicts a single subcarrier, it will be understood that multiple subcarriers may be modulated and detected. Tone modulation and detection 900 may be implemented using FM tone modulation, AM tone modulation, or combinations thereof. As shown, tone modulation and detection 900 may be used with DWDM in a defined optical grid, or with flex-grid optical transport networks that are enabled to transmit superchannels.

In tone modulation and detection 900, tone modulation is performed using optical transmitter 906, which may be an embodiment of transmitter 102 in FIG. 1. Thus, optical transmitter 906 may receive and modulate data onto the optical channel according to various types of modulation and carrier techniques described herein. Additionally, optical transmitter 906 may modulate tone 904 onto the optical channel. Tone 904 may represent an FM tone or an AM tone, as described previously. The optical channel may be combined with other optical channels to form the optical signal that is transmitted over optical transport network 902. As noted, tone 904 may be unique to a given optical channel within the optical signal or within optical transport network 902, which may represent at least certain portions of optical transport network 101 in FIG. 1.

At some location within optical transport network 902, optical channel monitor 920 may be employed to receive the optical signal and to detect and monitor individual optical channels. As shown, optical channel monitor 920 includes periodic optical filter 908 for spectral scanning using passbands 810, as described above with respect to FIG. 8. An output of periodic optical filter 908 is received by optical power sensor 910, which may represent a photodiode or a similar opto-electronic element, which outputs an electrical signal to analog-to-digital converter (ADC) 912. ADC 912 may digitize the electrical signal and send digital signals or data to periodic tone demodulator 914. Periodic tone demodulator 914 may include various combinations of hardware and software, such as a digital signal processor (DSP) and associated executable instructions, for demodulating tone 904 and generating a power spectra for each modulated frequency ν_(n) over the free spectral range W_(fs), as described with respect to FIG. 8. In some embodiments, an RF detector (not shown) may be used to detect the modulated frequencies ν_(n). It is noted that optical channel monitor 920 may communicate with optical transmitter 906 using a network control system, such as control system 700 in FIG. 7, for example, to receive modulation information, such as a frequency ν_(n) of tone 904. Because the frequency ν_(n) of tone 904 may be unique, detection of the frequency by optical channel monitor 920 may uniquely identify the optical channel. Conversely, when the unique frequency ν_(n) of tone 904 that is assigned to an optical channel is not detected by optical channel monitor 920, then a determination may be made that the optical channel is missing from the optical signal received by optical channel monitor 920.

Referring now to FIG. 10, a portion of optical spectrum 1000 is illustrated. Optical spectrum 1000 may represent a portion of an optical spectrum used for optical communications, such as the C-band. As shown, optical spectrum 1000 includes various optical carriers (or channels), shown at center frequencies f₁, f₂, f₃, and f₄ or respective optical bands, in an exemplary embodiment. In various embodiments, larger numbers of optical carriers may be allocated within optical spectrum 1000, as desired. In FIG. 10, the optical carriers are shown in a flex-grid with flexible allocation of optical bandwidth. Thus, the optical carriers shown in optical spectrum 1000 may include superchannels. In some embodiments, optical spectrum 1000 may be used for DWDM optical signals in a fixed grid arrangement.

In FIG. 10, a minimum bandwidth for an optical carrier is given by BW_(min), which may be a defined parameter for optical communications using optical spectrum 1000. The minimum bandwidth BW_(min) may correspond to a minimum baud rate of an optical carrier. Also, a maximum bandwidth for an optical carrier is given by BW_(max), which may correspond to a maximum baud rate of an optical carrier. It is noted that the value of W_(fs) may be selected to be at least BW_(max), for example, when a flex-grid optical network is used.

Also shown in optical spectrum 1000 are bandwidth slices associated with respective modulation frequencies, ν_(n), shown as ν₁, ν₂, ν₃, ν₄, ν₅, and ν₆, among others. A desired portion of optical spectrum 1000, such as the C-band, may be allocated to the bandwidth slices and corresponding modulation frequencies ν_(n). As shown, the bandwidth slices are defined with a fixed spectral band (or spectral width) B that is equal for each bandwidth slice. The equal spectral band B may be defined to be smaller than or equal to BW_(min). In this manner, any optical carrier that falls into one of the bandwidth slices B may be assigned with an associated modulation frequency ν_(n), which will be unique to the respective optical carrier. In this manner, when tone modulation is used with optical carriers, a unique modulation frequency ν_(n) may be assigned to each optical carrier.

After the optical carriers are modulated with a respective modulation frequency ν_(n), as described herein, the optical carriers may be transmitted over an optical transport network. When the optical carrier is received at a desired location from the optical transport network, an optical channel monitor may be used to demodulated the modulation frequency ν_(n) and to detect the optical carrier, as described with respect to FIG. 9, for example, including detecting a center frequency f_(n) of the optical carrier, such as using Equation 1 above. However, because the center frequency f_(n) of the optical carrier may be subject to a certain amount of frequency drift during transmission, Equation 1 may not yield the correct result for the center frequency f_(n). For example, when a periodic optical filter is used and a carrier center frequency f_(n) resides at an edge of the free spectral range W_(fs) when K is greater than or equal to 2, the frequency drift may cause the center frequency f_(n) to drift beyond an edge of the free spectral range W_(fs). In this situation, the detected value for ΔW_(n) may result in an incorrect value for the center frequency f_(n). However, using certain known properties of optical carriers, such as a maximum frequency drift that may occur during transmission, a method for obtaining an accurate actual center frequency f_(n) for an optical carrier may be employed, as will now be described in further detail.

Specifically, it may be assumed that a carrier frequency error such as caused by frequency drift during transmission, may be bounded by the fixed spectral band B. In other words, it may be assumed that frequency drift of an optical carrier during transmission is less than the fixed spectral band B. Therefore, the estimated center frequency f_(n), for example, as calculated using Equation 1 below, may be validated using Expression 1. f(ν_(n) ^(min))≦f _(n) ≦f(ν_(n) ^(max))  Expression (1) In Expression 1, f(ν_(n) ^(max)) is a maximum frequency and f(ν_(n) ^(min)) is a minimum frequency of the fixed spectral band B, related with ν_(n), respectively. The calculated carrier frequency f_(n) is valid when Expression 1 is satisfied. When the calculated carrier frequency f_(n) is not valid and Expression 1 is not satisfied, additional possible values of the center frequency, f_(n′) and f_(n″), may be calculated using Equations 2 and 3, for example using different values of m. f _(n′)=(m+1)*W _(fs) +W _(c1) +ΔW _(n)  Equation (2) f _(n″)=(m−1)*W _(fs) +W _(c1) +ΔW _(n)  Equation (3) Then, f_(n′) and f_(n″) may be validated using the following Expressions 2 and 3. |f _(n′) −f(ν_(n) ^(max))|≦w _(e) ^(max)  Expression (2) |f _(n″) −f(ν_(n) ^(min))|≦w _(e) ^(max)  Expression (3) In Expressions 2 and 3, w_(e) ^(max) is a maximum possible carrier frequency drift of a transmitter laser that may be smaller than B. In certain embodiments, a given value, such as 2.5 GHz, may be used for w_(e) ^(max).

Then, f_(n′) and f_(n″) may be evaluated using Equations 1 and 2 to determine whether at least one center frequency corresponds to a carrier frequency error that is less than w_(e) ^(max). A center frequency f, such as f_(n), f_(n′), or f_(n″), having a carrier frequency error according to Expressions 2 and 3 that is less than the maximum carrier frequency error, w_(e) ^(max) may be determined to be an actual center frequency for a carrier n. When no center frequency having a carrier frequency error according to Expressions 2 and 3 that is less than the maximum carrier frequency error, w_(e) ^(max), is determined, an error may be reported. It is noted that Expressions 2 and 3 may be applied singularly or in a different order, in different embodiments. In this manner, the spectral allocation of modulation frequencies ν_(n) using the bandwidth slices depicted in FIG. 10 may be performed over a desired region of optical spectrum 1000.

Referring now to FIG. 11, a method 1100 for using modulation frequencies is shown in flow chart form. As shown method 1100 may be performed using optical transport network 101 (see FIG. 1) in conjunction with optical channel monitor 920 (see FIG. 9) and optical control plane system 700 (see FIG. 7). In particular, method 1100 may be implemented using the carrier frequency error estimation described with respect to FIG. 10. It is noted that operations described with respect to method 1100 may be omitted or rearranged in different embodiments.

Method 1100 may begin by dividing (operation 1102) an optical band into bandwidth slices, each of the bandwidth slices having an equal spectral band that is smaller than a minimum optical carrier bandwidth. A modulation frequency that is respectively unique to each of the bandwidth slices may be assigned (operation 1104) to each of the bandwidth slices. The assignment of the modulation frequencies in operation 1104 may be recorded for subsequent access or reference, such as using database 704 (see FIG. 7). Based on a spectral position of a first bandwidth slice, a first optical carrier aligned with the spectral position may be assigned (operation 1106) to a first modulation frequency corresponding to the first bandwidth slice. The first optical carrier may be transmitted (operation 1108) over an optical transport network, including modulating the first optical carrier using a first modulation frequency, such that the first optical carrier is included with a plurality of optical carriers transmitted over the optical transport network. The optical carriers transmitted may be received (operation 1110) from the optical transport network. The optical carriers may be scanned (operation 1112) using a periodic optical filter over a free optical spectral range W_(fs) of the periodic optical filter to generate a filtered signal, such that the free optical spectral range W_(fs) may correspond to K number of optical carriers and K is greater than or equal to 2. When a flex-grid optical network is used, the free optical spectral range W_(fs) may correspond to at least a maximum optical carrier bandwidth. The filtered signal may be demodulated (operation 1114) to generate a power spectrum within the free spectral range for the first modulation frequency. Based on the first bandwidth slice and the power spectrum for the first modulation frequency, a first center optical frequency of the first optical carrier received may be identified (operation 1116). Operation 1116 may involve carrier frequency error analysis, as described above with respect to FIG. 10 using Expressions 1, 2, and 3, along with Equations 1, 2, and 3, as disclosed herein.

While the subject of this specification has been described in connection with one or more exemplary embodiments, it is not intended to limit any claims to the particular forms set forth. On the contrary, any claims directed to the present disclosure are intended to cover such alternatives, modifications and equivalents as may be included within their spirit and scope. 

What is claimed is:
 1. A method for using modulation frequencies, the method comprising: dividing an optical band into bandwidth slices, each of the bandwidth slices having an equal spectral band that is smaller than a minimum optical carrier bandwidth; assigning, to each of the bandwidth slices, a modulation frequency that is respectively unique to each of the bandwidth slices; based on a spectral position of a first bandwidth slice, assigning a first optical carrier aligned with the spectral position to a first modulation frequency corresponding to the first bandwidth slice; transmitting the first optical carrier over an optical transport network, including modulating the first optical carrier using the first modulation frequency, wherein the first optical carrier is included with a plurality of optical carriers transmitted over the optical transport network; receiving, from the optical transport network, the optical carriers transmitted; scanning the optical carriers using a periodic optical filter over a free optical spectral range W_(fs) of the periodic optical filter to generate a filtered signal, wherein the free optical spectral range W_(fs) corresponds to K number of optical carriers, and wherein K is greater than or equal to 2; demodulating the filtered signal to generate a power spectrum within the free spectral range for the first modulation frequency; and based on the first bandwidth slice and the power spectrum for the first modulation frequency, identifying a first center optical frequency of the first optical carrier received.
 2. The method of claim 1, wherein the first center optical frequency f_(n) for an optical carrier n is calculated using Equation (A): f _(n) =m*W _(fs) +W _(c1) +ΔW _(n)  Equation (A) wherein: m=floor((n−1)/K); W_(c1) is an initial scanning frequency of a first periodic passband of the periodic comb filter; and ΔW_(n) is an optical frequency shift with respect to W_(fs) at which a central alignment feature in the power spectrum is detected.
 3. The method of claim 2, wherein identifying the first center optical frequency further comprises: when a first carrier frequency error for the first center optical frequency is greater than the equal spectral band: calculating a second center optical frequency f_(n′) of the first optical carrier using Equation (B): f _(n′)=(m+1)*W _(fs) +W _(c1) +ΔW _(n)  Equation (B); and based on f_(n′), calculating a second carrier frequency error; and when the second carrier frequency error is less than the equal spectral band, determining that f_(n′) is an actual center optical frequency for the first optical carrier.
 4. The method of claim 2, wherein identifying the first center optical frequency further comprises: when a first carrier frequency error for the first center optical frequency is greater than the equal spectral band: calculating a third center optical frequency f_(n″) of the first optical carrier using Equation (C): f _(n″)=(m−1)*W _(fs) +W _(c1) +ΔW _(n)  Equation (C); and based on f_(n″), calculating a third carrier frequency error; and when the third carrier frequency error is less than the equal spectral band, determining that f_(n″) is the actual center optical frequency for the first optical carrier.
 5. The method of claim 2, wherein a carrier frequency error for a carrier frequency f is calculated using at least one of: an absolute value of a difference between f and a maximum frequency of the bandwidth slice corresponding to the modulation frequency f; and an absolute value of a difference between f and a minimum frequency of the bandwidth slice corresponding to the modulation frequency f.
 6. The method of claim 1, wherein the first optical carrier is modulated using frequency modulation (FM), and wherein the central alignment feature is a relative minima in the power spectrum.
 7. The method of claim 1, wherein the first optical carrier is modulated using amplitude modulation (AM), and wherein the central alignment feature is a relative maxima in the power spectrum.
 8. The method of claim 7, wherein demodulating the filtered signal includes: using a digital signal processor to detect at least the first modulation frequency.
 9. The method of claim 1, wherein demodulating the filtered signal includes: when the first modulation frequency is not detected from the filtered signal, determining that the first optical carrier is not included in the optical carriers received.
 10. The method of claim 1, wherein the optical carriers are subcarriers of a superchannel, and wherein the free optical spectral range W_(fs) corresponds to at least a maximum optical carrier bandwidth.
 11. An optical transport network enabled for using modulation frequencies, the optical transport network further comprising: a control system enabled to: divide an optical band into bandwidth slices, each of the bandwidth slices having an equal spectral band that is smaller than a minimum optical carrier bandwidth; assign, to each of the bandwidth slices, a modulation frequency that is respectively unique to each of the bandwidth slices; and based on a spectral position of a first bandwidth slice, assign a first optical carrier aligned with the spectral position to a first modulation frequency corresponding to the first bandwidth slice; a transmitter enabled to: transmit the first optical carrier over the optical transport network, including modulating the first optical carrier using the first modulation frequency, wherein the first optical carrier is included with a plurality of optical carriers transmitted over the optical transport network; and an optical channel monitor enabled to: receive the optical carriers transmitted over at least a portion of the optical transport network, scan the optical carriers using a periodic optical filter over a free optical spectral range W_(fs) of the periodic optical filter to generate a filtered signal, wherein the free optical spectral range W_(fs) corresponds to K number of optical carriers, and wherein K is greater than or equal to 2; demodulate the filtered signal to generate a power spectrum within the free spectral range for the first modulation frequency; and based on the first bandwidth slice and the power spectrum for the first modulation frequency, identify a first center optical frequency of the first optical carrier received.
 12. The optical transport network of claim 11, wherein the optical channel monitor calculates the first center optical frequency f_(n) for an optical carrier n using Equation (A): f _(n) =m*W _(fs) +W _(c1) +ΔW _(n)  Equation (A) wherein: m=floor ((n−1)/K); W_(c1) is an initial scanning frequency of a first periodic passband of the periodic comb filter; and ΔW_(n) is an optical frequency shift with respect to W_(fs) at which a central alignment feature in the power spectrum is detected.
 13. The optical transport network of claim 12, wherein the optical channel monitor identifying the first center optical frequency further comprises: when a first carrier frequency error for the first center optical frequency is greater than the equal spectral band: calculating a second center optical frequency f_(n′) of the first optical carrier using Equation (B): f _(n′)=(m+1)*W _(fs) +W _(c1) +ΔW _(n)  Equation (B); and based on f_(n′), calculating a second carrier frequency error; and when the second carrier frequency error is less than the equal spectral band, determining that f_(n′) is an actual center optical frequency for the first optical carrier.
 14. The optical transport network of claim 12, wherein the optical channel monitor identifying the first center optical frequency further comprises: when a first carrier frequency error for the first center optical frequency is greater than the equal spectral band: calculating a third center optical frequency f_(n″) of the first optical carrier using Equation (C): f _(n″)=(m−1)*W _(fs) +W _(c1) +ΔW _(n)  Equation (C); and based on f_(n″), calculating a third carrier frequency error; and when the third carrier frequency error is less than the equal spectral band, determining that f_(n″) is the actual center optical frequency for the first optical carrier.
 15. The optical transport network of claim 13, wherein a carrier frequency error for a carrier frequency f is calculated using at least one of: an absolute value of a difference between f and a maximum frequency of the bandwidth slice corresponding to the modulation frequency f; and an absolute value of a difference between f and a minimum frequency of the bandwidth slice corresponding to the modulation frequency f.
 16. The optical transport network of claim 11, wherein the first optical carrier is modulated using frequency modulation (FM), and wherein the central alignment feature is a relative minima in the power spectrum.
 17. The optical transport network of claim 11, wherein the first optical carrier is modulated using amplitude modulation (AM), and wherein the central alignment feature is a relative maxima in the power spectrum.
 18. The optical transport network of claim 17, wherein the optical channel monitor demodulating the filtered signal further comprises: using a digital signal processor to detect at least the first modulation frequency.
 19. The optical transport network of claim 11, wherein the optical channel monitor demodulating the filtered signal further comprises: when the first modulation frequency is not detected from the filtered signal, determining that the first optical carrier is not included in the optical carriers received.
 20. The optical transport network of claim 11, wherein the optical carriers are subcarriers of a superchannel, and wherein the free optical spectral range W_(fs) corresponds to at least a maximum optical carrier bandwidth. 